Method and apparatus for transmitting messages in a telecommunication system

ABSTRACT

A method and an apparatus for implementing the method, which allow a transmitter ( 100 ) of a digital telecommunication system transmitting speech to transmit predetermined messages to a receiver ( 102 ) over a speech channel ( 108 ). Messages ( 114 ) are transmitted from the transmitter ( 100 ) to the receiver ( 102 ) on the speech channel ( 108 ) in such a way that they are encoded in some of the speech coding bits, whereby the speech coding bits selected for message transmission have been selected in such a way that they affect the quality of speech as little as possible. Most preferably, the speech coding bits used for transmitting messages are protected by channel coding ( 702, 704 ). Speech coding bits are “stolen” for message transmission only for very short periods at a time and only for exactly the time it takes to transmit the messages ( 114 ); at other times, the entire speech channel ( 108 ) is used normally for speech transmission.

This application is the national phase of international applicationPCT/F196/00579 filed Oct. 30, 1996 which designated the U.S.

FIELD OF THE INVENTION

Background of the Invention

The invention relates to a method and an apparatus which allow atransmitter of a digital telecommunication system transmitting speech totransmit predetermined messages to a receiver on a speech channel. Inmany digital telecommunication systems it is necessary to transmit notonly encoded speech but also messages containing other information, e.g.messages pertaining to the control of a speech connection or messagescontaining data completely independent of speech. Such messages areoften called signalling.

In telecommunication systems transmitting speech, a speech signal isusually subjected to two coding operations: speech coding and channelcoding. Speech coding comprises speech encoding performed in atransmitter by a speech encoder, and speech decoding performed in areceiver by a speech decoder.

With reference to FIG. 1, a speech encoder 106 located in a transmitter100 compresses a speech signal so that the number of bits used torepresent it per unit of time is reduced. The speech encoder 106typically processes speech as speech frames containing a certain amountof speech samples. On the basis of sampled speech, the speech encoder106 calculates speech parameters, each of which is encoded as a separatebinary code word. The speech parameters produced by the RPE-LTP speechencoder used in the full-rate channel of the pan-European GSM mobiletelephone system are described in ETSI GSM Recommendation 06.10. Theseparameters are also disclosed in Table 1 of Appendix 1. The RPE-LTP(Regular Pulse Excitation—Long Term Prediction) produces 76 speechparameters from one speech frame of 20 ms (corresponding to 160 speechsamples at a sampling frequency of 8 kHz). Recommendation GSM 06.10 alsodiscloses the length of the binary code word assigned for eachparameter.

Very often speech encoders also group speech parameters together, inwhich case each group—instead of a single speech parameter—is encoded toa separate code word. Encoding parameters in groups is called vectorquantization. Modern speech encoders usually encode some speechparameters separately and some in groups (the RPE-LTP speech encoder ofthe example does not employ vector quantization). The result produced bythe speech encoder is thus a constant-speed bit stream. The RPE-LTPspeech encoder of the invention produces 260 speech coding bits per eachspeech frame of 20 ms.

The speech decoder 110 of a receiver 102 performs a reverse operationand synthesizes a speech signal 112 from the bits produced by the speechencoder. The decoder 110 receives binary code words and generatescorresponding speech parameters on the basis of them. The synthesizationis performed by the use of the decoded speech parameters. The speechsynthesized in the receiver is, however, not identical with the originalspeech compressed by the speech encoder, but it has changed more or lessas a result of the speech coding. The higher the degree of compressionused in the speech coding, the more the quality of speech usuallydeteriorates in the coding process.

The RPE-LTP speech encoder, for example, compresses a speech signal to arate of 13 000 bits per second (13 kbps). The compression is performedin such a way that it affects the intelligibility of speech as little aspossible. In special cases, such as identification of tone pairs used intone dialling, the compression may detrimentally affect or evencompletely obstruct the process.

The above-mentioned channel coding comprises channel encoding performedin the transmitter by a channel encoder, and channel decoding performedin the receiver by a channel decoder. The purpose of channel coding isto protect speech coding bits to be transmitted against errors occurringon the transmission channel. Channel coding may either allowtransmission errors to be detected without being able to correct them orit may allow transmission errors to be corrected, provided that thenumber of errors is smaller than a certain maximum number, which isdependent on the channel coding method.

The channel coding method to be used is selected according to thequality of the transmission channel. In fixed transmission methods, theerror probability is often very small, and there is not much need forchannel coding. In wireless networks such as mobile telephone networks,however, the error probability is often extremely high, and the channelcoding method employed has a significant effect on the quality ofspeech. In mobile telephone networks, both error-detecting anderror-correcting channel coding methods are usually employedsimultaneously.

In telecommunication systems transmitting speech, speech coding andchannel coding are closely connected. The importance of bits produced bya speech encoder for the quality of speech usually varies such that, insome cases, an error in an important bit may cause an audibledisturbance in synthesized speech, whereas several errors in lessimportant bits may be almost imperceptible. How great the differencebetween the importance of speech coding bits is depends essentially onthe speech coding method employed, but at least small differences can befound in most methods. When a speech transmission method is developedfor a telecommunication system, channel coding is thus designed togetherwith speech coding to allow the bits that are the most important for thequality of speech to be better protected than less important bits. In afull-rate channel of the GSM, for example, the bits produced by anRPE-LTP speech encoder have been divided into three different classesaccording to their importance to channel coding: the most importantclass is protected in channel coding with both an error-correcting andan error-detecting code; the second most important class is protectedonly with an error-correcting code; and the least important class is notprotected in channel coding at all. Table 2 of Appendix 1 shows theclassification of bits produced by an RPE-LTP encoder in two differentways: 6-parted subjective classification, and 3-parted classificationused by channel coding.

Channel coding is not directly relevant to the principle of theinvention. In view of speech coding, channel coding is part of thetransmission channel. In view of the practical implementation, channelcoding is, however, of essential significance to the transmission ofmessages as regards the selection of bits, as will be seen from theexamples below.

The term “channel” can be interpreted in many ways in the field,wherefore the meaning of the term for the present invention can bespecified as follows. When messages and speech are transmitted onseparate channels, the receiver can distinguish between message bits andspeech coding bits irrespective of the contents of the informationtransmitted on the channels. However, two channels are not necessarilyphysically separate channels. Separate channels can also be provided bydividing one physical transmission channel (e.g. a radio path or atransmission line) into a plurality of time slots and frequency ranges.When such a division is made unambiguously, the receiver can distinguishbetween the channels irrespective of the contents of the informationtransmitted on them.

The methods of FIGS. 1 to 3 are employed for transmitting messagessubstantially simultaneously with speech. The methods will be consideredfrom four points of view. 1: Is a separate transmission channel neededfor transmitting messages, or can messages be transmitted on the samechannel as speech? 2: How does the transmitter of the message have tocommunicate to the receiver that a message is on its way? 3: How doesthe transmission of a message affect the quality of speech transmittedsimultaneously? 4: What happens in an old receiver if a new transmittertransmits a message and the message transmission method is notimplemented in the old receiver?

FIG. 1 illustrates message transmission most generally known in the art.FIG. 1 shows both a transmitter 100 and a receiver 102. In thisarrangement, messages and speech are transmitted on completely differentchannels. In the transmitter 100, a digital speech signal 104 issupplied to a speech encoder 106, which, from this signal, generatescompressed speech coding bits, which are sent to the receiver on aspeech channel 108. In the transmitter, a message 114 to be sent to thereceiver is supplied to a message encoder 116, which generates messagebits, which are then sent to the receiver on a separate message channel118. The receiver 102 receives the speech coding bits from a speechchannel 108 and supplies them to a speech decoder 110, which synthesizesthe speech signal 112 to be heard. The receiver 102 receives the messagebits from a separate message channel 118 and supplies them to a messagedecoder 120, which interprets the transmitted message 122.

This arrangement is used, for example, in the SACCH (Slow AssociatedControl Channel) of the GSM system. A GSM speech channel is alwaysconnected to a separate SACCH, on which messages relating to the controlof the speech channel are transmitted.

A typical feature of the prior art method shown in FIG. 1 is that whenmessages and speech are transmitted simultaneously in the system, thecombined transmission capacity used for the transmission of messages andspeech is always greater than the transmission capacity used for thetransmission of speech.

Transmission of messages thus does not affect transmission of speech inany way.

In the light of the above-mentioned viewpoints, the message transmissionillustrated in FIG. 1 can be described as follows. 1: Messages andspeech are transmitted on different channels. 2: Because a separatechannel exists, such a communication is not needed, for all informationtransmitted on the message channel is message information. 3: A messagedoes not affect the quality of speech. 4: The method of FIG. 1 cannotnormally be introduced at all in an existing system, since it is usuallyimpossible to add a separate channel to the system.

FIG. 2 illustrates a second type of message transmission generally knownin the art. The parts in FIGS. 2 to 8 that have the same referencenumerals as in FIG. 1 also have the same function, wherefore they willnot be described again. In the arrangement of FIG. 2, the speech encoder106 and the message encoder 116 use the same transmission channel 108 atdifferent times. The channel 108 can be used for transmitting messages114 only for short periods at a time (typically 10 to 30 ms at a time),since transmission of speech 104 must be interrupted for this period,and a long interruption would significantly deteriorate the quality ofspeech.

When the channel 108 is used for transmitting speech, a transmissionswitch 202 provided in the transmitter 100 is arranged to allow speechcoding bits to be supplied to the channel 108. When messages 114 aretransmitted, the switch 202 is arranged to allow message bits to besupplied to the channel 108 instead of speech coding bits. Thus, onlyspeech or message bits are transmitted on the channel 108 at a certainmoment of time. The transmitter 100 must therefore separately inform thereceiver 102 of whether messages or speech is transmitted on the channel108. In the transmitter 100 the information 206 is supplied to aseparate channel 208. On the basis of this information 210, the receiverplaces switch 204 in a position that corresponds to that of switch 202.The bits are received from the channel 108 and supplied, according tothe position of the switch 204, either to the speech decoder 110 or tothe message decoder 120.

This procedure is employed, for example, with a FACCH (Fast AssociatedControl Channel) of the GSM system. A FACCH is used, for instance, fortransmitting messages relating to the management of a speech channel. Isshould be noted that a FACCH is not a “channel” in the same sense thatthe term is understood in connection with the present application. Thisis because the messages of a FACCH are transmitted on a speech channelas shown in FIG. 2. One bit is always transmitted per one GSM speechframe on a completely different channel to indicate whether the framecontains speech coding bits (normal case) or whether the bits have been“stolen” for message transmission.

In the light of the above-mentioned viewpoints, the message transmissionof FIG. 2 can be described as follows. 1: Messages and speech aretransmitted on the same channel at different times. The time used formessage transmission is very short as compared with the time used fortransmission of speech. 2: A communication about whether the channel isused for transmitting speech or messages is sent on a separate channel.3: A message deteriorates the quality of speech, but only to a verysmall extent (almost insignificantly). 4: Normally the method cannot beintroduced at all in an existing system, as it requires a separatechannel for communicating the selection between speech and messages, andit is usually impossible to add such a channel to the system.

FIG. 3 illustrates a third arrangement known in the art, which isspecified for simultaneous transmission of data and speech, for example,in recommendations G.722 and G.727 of the former CCITT (presentITU-TSS). These recommendations are descriptions of ADPCM speech codingmethods. The method is employed for providing a fixed data channel foruse with a speech channel. A speech encoder 106 and a message encoder116 use the same transmission channel 108 simultaneously in such a waythat bits from speech coding are “stolen” for message transmission. Amessage decoder 120 of the receiver 102 removes the message bits fromthe speech coding bits, interprets the message 122, and supplies theremaining speech coding bits to the speech decoder 110. The speechdecoder 110 synthesizes the speech 112 to be heard. The speech decoder110 must also know which received bits are speech coding bits and whichare message bits. An erroneous interpretation deteriorates the qualityof speech significantly.

In this arrangement, the message encoder 116 replaces some of the speechcoding bits with bits used for coding the message 114. In ADPCM coding,the bits to be removed can be selected easily such that they have aslittle effect as possible on the speech quality to be obtained. ADPCMcoding uses code words of fixed length, the least important bits ofwhich are replaced with message bits. An ADPCM speech encoder is thusnormally designed in such a way that the possibility of stealing bits istaken into account and the effect of such stealing on the quality ofspeech is already minimized in the encoding method.

Even the arrangement shown in FIG. 3 requires information on which ofthe bits transmitted on the transmission channel 108 are message bits.This information 206 is sent from the transmitter through a separatechannel 208 to the receiver, where the received bits are divided bymeans of this information 210 into messages 122 and speech 112.

Only little signalling information is required on channel 208, since thesame channel division between speech coding and data is maintained for along time.

In the light of the above-mentioned viewpoints, the arrangement of FIG.3 can be described as follows. 1: Messages and speech are transmitted onthe same channel simultaneously. 2: A communication on whether speech ormessages are transmitted on the channel is sent on a separate channel.However, the communication is typically valid for a long time (oftenduring an entire connection), wherefore it requires only very littlecapacity. 3: A message deteriorates the quality of speech, sincemessages typically reserve part of the channel permanently for theiruse. 4: Normally the method cannot be introduced at all in an existingsystem, for it requires a separate channel for informing of theselection between speech and messages, and it is usually impossible toadd such a channel to the system.

It is also known to use the same physical transmission channel fortransmitting both speech and digital information. U.S. Pat. No.4,476,559 (Brolin et al.), for example, discloses such a technique foruse in a fixed network. This technique comprises selecting one of threetransmission forms (speech, data or their combination), and providingfor each transmission form a “ounique signature” (using the terminologyof the cited U.S. patent) which is interleaved between transmissionsignals to indicate the transmission form. However, there are severalreasons why the technique disclosed in the U.S. Patent (Brolin) is notsuitable for an Ad environment to which the present invention is to beapplied. First, according to the U.S. Patent (Brolin), part of thebandwidth is continuously reserved for indicating the selectedtransmission form, wherefore the entire bandwidth cannot be used fortransmission of speech even when there are no messages to betransmitted. In a mobile communication system, and particularly at itsair interface, this would be too strict a restriction. Second, accordingto the U.S. Patent (Brolin), it is assumed that the “unique signature”indicating the transmission form can always be received without anyerrors. In the case of telecommunication over a radio interface, such anassumption cannot be made.

A problem arises when the telecommunication system is to be changed in away that has not been anticipated when the system was planned. Let usassume, for example, that more than two speech codecs are to be used ina GSM system. Signalling for this kind of selection has not beendesigned in the system, and if it is designed afterwards, it cannot beimplemented in old equipments that are already in use. To solve thisproblem, it is necessary to have a signalling method which can beintroduced into an existing telecommunication system without disturbingthose equipment in use in which this signalling method is notimplemented. Using such a method, new equipments can signal with eachother to agree on the use of a new codec; the signalling will notsucceed with old equipment, and thus the new equipment can conclude thatthe old speech codec must be used on the connection. Messagetransmission methods previously used in the field do not usually allowmessages to be added to an existing system.

It is possible to design various signalling possibilities forunpredictable cases in advance. If such a signalling possibility exists,it should be used primarily. However, such reserve signalling does notoften exist or its introduction may require a time-consumingstandardization process. Since there is, in any case, a limited numberof reserve signalling possibilities, such signalling cannot beintroduced very lightly.

An example of signalling that is designed in advance is the use of aspeech coding method. Since the speech encoder of the transmitter andthe speech decoder of the receiver must use the same speech codingmethod, the equipments must agree on the method to be used when thespeech connection is being established. Such a situation will arise, forexample, in the GSM system, where a half-rate speech codec will soon beintroduced in addition to the full-rate speech codec. In the GSM system,the problem of selecting the speech codec has been solved in such a waythat when the system was planned, it was already known that there wouldbe two speech codecs even though only one of them is implemented in thepresent equipments. A signalling method for selecting the speech codechas already been designed in the system in advance. The signalling isimplemented in the present equipments, and when new equipments with twospeech codecs are introduced later, the new equipments can use the oldspeech codec when communicating with the old equipments, since theselection of a speech codec is implemented in both the old and the newequipments.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is thus to provide a method which allows newfunctional properties to be added to an existing telecommunicationsystem intended for transmission of speech by disturbing the equipmentsalready in use in the system (“old” equipments) as little as possible.In addition, the object of the invention is to provide an apparatus forimplementing this method.

The invention is based on the idea that messages are transmitted from atransmitter to a receiver on a speech channel, encoded in speech codingbits selected such that the quality of speech is affected as little aspossible. Speech coding bits are “stolen” for message transmission onlyfor short periods at a time, and only for precisely the time it takes totransfer the messages; at other times, the entire speech channel is usednormally for transmission of speech.

An advantage of the signalling method of the invention is that it allowsnew properties to be added to existing telecommunication systems. Asystem may comprise both “new” equipments (in which the signallingmethod of the invention is implemented) and “old” equipments (in whichthe method is not implemented). When a new equipment communicates withanother new equipment, messages associated with the method aretransmitted between the transmitter and the receiver without disturbingthe speech connection. When a new equipment communicates with an oldequipment, the messages transmitted by the new equipment are notreceived, but neither is the speech connection disturbed. A receiveremploying the method of the invention can detect a message coded amongspeech coding bits and interpret it without that the speech connectionis essentially disturbed; no further information is required fordetecting the message. No specific speech frame corresponding to the“unique signature” of the above-mentioned U.S. Pat. No. 4,476,559(Brolin) is thus required in the present invention to indicate whetherinformation on the channel is to be interpreted as speech or as amessage. A receiver in which the message transmission system of theinvention is not implemented cannot detect a message coded among speechcoding bits, but the existence of the message does not essentiallydisturb the speech connection.

DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail bymeans of preferred embodiments with reference to the accompanyingdrawings, in which

FIGS. 1 to 3 illustrate different known arrangements for transmittingmessages and speech substantially simultaneously;

FIG. 4 illustrates a simple arrangement of the invention fortransmitting a message;

FIGS. 5 to 8 illustrate preferred embodiments of the invention.

DETAILED DESCRIPTION

With reference to FIG. 4, a digital speech signal 104 is supplied in atransmitter 100 to a speech encoder 106, which produces speech codingbits which are supplied to a message encoder 116. The message encoder116 replaces some of the speech coding bits with bits used for codingthe message 114. The bits to be replaced are selected according to thespeech coding method in such a way that the quality of speech isaffected as little as possible. The bits selected to be replaced arealways the same in the method, and the message decoder must be informedof them. The message encoder thus replaces the selected speech codingbits with message bits only when there is a message to be transmitted.Most of the time, only speech coding bits are transmitted on the channel108.

From channel 108, a bit stream is supplied to a message decoder 120 ofthe receiver 102. The decoder examines the bits of the received bitstream that have been selected to be replaced, and on the basis of theircontents concludes whether the bit stream includes a message. How thisconclusion is made will be described later. If a message is detected,the message decoder 120 interprets the message 122. The received bitsare forwarded to a speech decoder 110 as such, irrespective of whetherthey contain a message or not. The speech decoder 110 synthesizes thespeech 112 to be heard. During the message, the speech coding bitsselected to be replaced receive incorrect values in view of speechcoding, which deteriorates the quality of synthesized speech. However,the quality deteriorates only minimally when the messages are short, andthe bits to be replaced are correctly selected.

The method of the invention is suitable for use in applications wherethere is relatively little need for transmitting messages. The intervalbetween successive messages should typically be at least about a secondto prevent the quality of speech from deteriorating. Temporarily and insome special cases, messages can be sent even more frequently. Themethod is thus not suitable for providing a common data channel as theprior art methods described above. However, the method is well suitedfor transmitting control information and small amounts of data.

How a message decoder can conclude whether the received bits contain amessage will be described in the following. The number of differentpossible messages is very small, e.g. about 20. The selected code wordsused for coding messages can, however, be very long, e.g. about 100bits. Normally, a code word of 100 bits can be used for coding 2¹⁰⁰different messages; in view of this, such a code word is thus muchlonger than necessary. An advantage of a long code word is that it makesit impossible in practice for a speech encoder to produce one of the 20selected message code words of e.g. 100 bits by chance. The messagedecoder 120 can therefore examine the 100 bits used for message coding,and if they form one of the code words, they are interpreted as amessage. Otherwise the bits are assumed to be normal speech coding bits.No special signal is therefore required to indicate whether a message istransmitted or not.

The bits selected for message coding thus depend on the speech codingmethod employed. Table 3 of Appendix 1 shows, by way of example, how thebits can be selected in the case of an RPE-LTP speech coding method of aGSM full-rate channel. The selected bits are indicated in Table 3 bydouble framing. The numbering in Table 3 corresponds to the numbering ofbits in Table 1. From a speech frame of 260 bits, 123 bits have beenselected for message coding. Several factors have an effect on theselection. As a whole, the selection must be made in such a way thatthere is a sufficient number of bits but that they do not affect thequality of speech too much.

The above-mentioned message coding bits are selected on the followinggrounds. The selected 123 bits are all bits used for describing RPEparameters, and they form a uniform, sufficiently large group. The bitsof RPE parameters which belong to class 2 in channel coding have beencompletely excluded, since they are not protected at all by channelcoding, and their error probability in transmission is too high. Allbits of the RPE parameters of classes 1 a and 1 b (RPE grid position,block amplitude, RPE pulses) have been selected for message coding,which gives the number 123. The effect of these parameters on thequality of speech can be minimized by setting the values of blockamplitude parameters in all message code words as zero. The values ofthese parameters have a direct effect on how the other RPE parameters(RPE grid position and RPE pulses) affect the quality of speech. Whenthe block amplitude is set as zero, the incorrect values of the otherparameters no longer have any greater effect. The incorrect value (i.e.zero) of the block amplitude parameters thus slightly deteriorates thequality of speech in the receiver, but this is controlled attenuationwhich can be kept almost unnoticeable by using short messages (by notencoding messages in successive speech frames). There are altogether 20block amplitude bits in classes 1 a and 1 b, wherefore 103 bits are leftfor the actual message coding. The main reasons for the selection isthus that the selected bits are protected by channel coding, and thatthe use of the block amplitude parameter allows the effect of this bitgroup on the quality of speech to be minimized.

In the light of the viewpoints disclosed in connection with FIG. 1, thearrangement of FIG. 4 can be described as follows. 1: Messages andspeech are transmitted on the same channel simultaneously. Messages aretransferred only when there are messages to be transmitted in thetransmitter. 2: A message decoder concludes from a received bit streamwhether it contains a message. No additional signalling is required forthis purpose. 3: A message deteriorates the quality of speech only verylittle, since the messages are short, and the bits used for messagetransmission have been selected in such a way that they have as littleeffect on the quality of speech as possible. 4: A receiver, which doesnot comprise a message decoder, is hardly disturbed at all by messages.Messages are detected by the receiver as transmission errors, and whenthere is a sufficiently small number of them, they have very littleeffect. The method can be introduced into an existing system.

As can be seen from the above description, “simultaneous” transmissionof speech and messages on the same channel refers to transmittingmessages in bits normally used by speech but in a manner in which thereis very little effect on the speech as heard by a listener.

It can be seen that, of the known techniques, the ADPCM signalling ofFIG. 3 is most closely related to the method of the invention in thesense that both these methods comprise borrowing bits of the speechchannel for transmitting messages. An essential difference between thesetwo techniques is that in the method of the invention the additionalchannel 208 shown in FIGS. 1 to 3 is not needed at all.

FIG. 5 illustrates an alternative embodiment of the transmitter. Thedifference between this embodiment and the one of FIG. 4 is that thereis a feedback 502 from the message encoder 116 to the speech encoder106. The feedback 502 has the following function: The speech encodersusually function as state machines in such a way that, as a result of aperformed speech encoding, the speech encoder 106 is in a certain kindof internal state. This state has an effect on the further operation ofthe encoder. The speech decoder 110 operates in the same way, and if noerrors occur during the transmission, the states of the encoder 106 andthe decoder 110 correspond to each other. On account of transmissionerrors, the state of the decoder 110 may differ from that of the encoder106, wherefore the quality of the synthesized speech 112 deteriorates.Since the bits stolen by the message encoder 116 cause “transmissionerrors” from the point of view of the speech decoder 110 of thereceiver, the states of the speech encoder 106 and the speech decoder110 will differ from each other in this way. The difference usually doesnot affect the quality of speech to any essential degree, but somespeech coding methods may have even a great effect. In the embodimentshown in FIG. 5, the discrepancy between the states of the encoder 106and the decoder 110 can, however, be prevented by feeding the changescaused by the message encoder 116 in the speech coding bits back to thespeech encoder 106, which will change its state on the basis of them. Asthe state of the speech decoder 110 is also determined on the basis ofthe bits produced by the message encoder 116, the states will not differfrom each other.

FIG. 6 illustrates an alternative embodiment of the receiver, by whichany deterioration of the quality of speech caused by the messages can bereduced in the receiver of the invention. This embodiment differs fromthe one of FIG. 4 only as regards the receiver. After the messagedecoder 120 has interpreted a message in the receiver 102, the receivedbits are supplied to a block 602 which replaces bad parameters. If thespeech frame contains a message, the receiver 102 knows that some of thereceived bits are erroneous. The receiver 102 also knows which of thebits are possibly erroneous, since both the transmitter 100 and thereceiver 102 know which bits have been selected for message coding. As aresult of the erroneous bits, the corresponding decoded parameters willalso be erroneous. Instead of supplying bad parameters to the speechdecoder 110 as such, they can be replaced with others, using a methodknown per se for replacing bad parameters. Such methods are usuallybased on replacing bad parameters by the use of the same error-freeparameters received previously. The method of replacement can beemployed in accordance with FIG. 6 if the selection of message codingbits cannot alone eliminate the deteriorating effect of the message onthe quality of speech.

When the same system comprises both “new” and “old” equipments, thesystem will operate as follows. Let us assume at first that thetransmitter is “new” and the receiver is “old”. Such a situation wouldresult if the message decoder 120 in FIG. 4 were removed. As stated inconnection with FIG. 4, the transmitter 100 codes the speech and, ifnecessary, replaces some of the speech coding bits with bits used forcoding the message 114. The “old” receiver 102 does not comprise amessage decoder 120, and the bits are supplied directly to a speechdecoder 110, which synthesizes the speech 112 to be heard. If the speechcoding bits contain a message, the message will deteriorate the qualityof speech to some extent. If the message bits have been selectedappropriately, the quality will deteriorate only minimally. The receiver102 does not understand the message, but nor is the speech connectionessentially disturbed.

In the reverse case, the transmitter is “old” and the receiver is “new”,whereby the system will operate as follows. The transmitter 100 codesthe speech as described above and supplies it to a channel 108. An “old”transmitter does not replace speech bits with message bits. The messagedecoder 120 of the receiver 102 examines whether the received bitscontain a message. Since the message code words are very long, thespeech coding bits cannot in practice contain a message by chance,wherefore the message decoder 120 supplies the bits as such to a speechdecoder 110, which synthesizes the speech 112 to be heard. The messagedecoder 120 thus never interprets the message 122. The receiver 102operates in this case as a normal receiver in which the messagetransmission method in question is not implemented.

FIG. 7 illustrates the method of the invention in an environment inwhich channel coding is also visible. If the method is employed in amobile telephone system, channel coding is always an essential factorfor the operation of the system. Channel coding has no effect on thelogical operation of the method, but FIG. 7 shows how channel coding isarranged in the method of the invention. Speech coding bits, possiblycontaining message bits, are supplied from the message encoder 120 to achannel encoder 702, where the bits are protected against errorsoccurring on the channel 108. In the receiver 102, the bits are at firstsupplied to a channel decoder 704, which decodes the channel encoding.After that, the bits ,are supplied, to the, message decoder 120, and theoperation continues as in the case of FIG. 4.

Transmission errors possibly occurring on the channel 108 have also aneffect on the implementation of the present invention. If theprobability of transmission errors exists, the reception of messages canbe jeopardized. If the error probability of the channel is, for example,1%, a message code word of 100 bits often contains one or more errors.This can be taken into account in the operation of the message decoder.It is usually not sensible to implement a message decoder in such amanner that it interprets a message as detected only if it is exactlyidentical with a predetermined message code word. In the practicalimplementation, a message decoder allows a certain number of errors in amessage code word (e.g. up to 3 errors). If a received bit patterndiffers from a message code word by not more than the selected maximumnumber of errors, the message is interpreted as detected. The number ofdifferent bits is called the Hamming distance. Such a procedureincreases the risk that a bit combination resembling a message code wordmight occur by chance among speech coding bits. It is, however, possibleto find a satisfying compromise by selecting sufficiently long messagecode words and a sufficiently small number of allowed errors.

In the following, some examples are given of how the messagetransmission of the invention can be utilized in a mobile telephonesystem. An extensive application utilizing the present invention isdisclosed in the Applicant's copending Finnish Patent Application No.955267 with the same filing date.

An embodiment is the transmission of DTMF sounds in a GSM system. DTMF(Dual Tone Multi-Frequency) sounds refer to signalling sounds which areused in a public switched telephone network (PSTN) and which areproduced by pushing the buttons in a voice-frequency push-buttontelephone. See e.g. Recommendation T/CS 46-02 (Innsbruck 1981, revisedat Nice 1985), “Signalling system for push-button telephones combiningbasic multifrequency signalling with direct current signalling.”

DTMF sounds propagate in the network “acoustically” along a speechchannel. Several automatic services offered by a telephone network arecontrolled by DTMF sounds. DTMF sounds are often also used, for example,for remote unloading a telephone answering machine. The control functionof DTMF sounds is based on the fact that their exact form is defined,and the receiver can identify and distinguish between various DTMFsounds and function according to a predetermined practice. There are 16different DTMF sounds.

The transmission of DTMF sounds in digital mobile telephone networkscauses major problems, since the speech coding methods used in thesenetworks are designed for transmission of speech and they distortsignalling sounds such as the DTMF sounds. If the network sends a DTMFsignal to a GSM mobile telephone (downlink direction), the sound can nolonger be reliably identified as a DTMF sound after speech coding. Inthe GSM, it is possible to transmit DTMF sounds on a separate signallingchannel towards the network, but not in the downlink direction.

FIG. 8 illustrates how the method of the invention allows DTMF sounds tobe reliably transmitted in the downlink direction in a GSM full-ratechannel. In this case, the transmitter 100 of FIG. 8 corresponds to atranscoder of a GSM network and the receiver 102 to a GSM mobilestation.

The DTMF signalling system of the invention requires 16 message codewords to be transmitted in an RPE-LTP speech frame. The message codeword bits selected can be the 123 bits shown in Table 4 of Appendix 1.The first row of Table 4 shows the consecutive numbering of the messagecode word bits. The second row indicates the serial number of thecorresponding message code word bit in the RPE-LTP speech frame (seeTable 3). The rest of the table comprises 16 rows, one per each messagecode word. Each code word is 123 bits long, and the code words aredivided into four sub-tables. Of these bits, 20 (the block amplitudebits) are set as zero in all the 16 code words. In Table 4 these bitsare in bold type. As regards the other 103 bits, the code words can beselected randomly; it should, however, be noted that the code words mustbe sufficiently different from one another. Table 4 shows the 16 codewords of 123 bits chosen for this example. The code words have beenchosen in such a way that they all differ from one another by at least35 bits. Thus there is in practice no risk of the code words gettingmixed up. The code words of Table 4 are thus predetermined and knownboth at the transmitter and at the receiver.

An audio signal 104 (which may be speech or a DTMF signalling sound) issupplied in the transmitter 100 to a DTMF detector 802, which examineswhether the signal is one of the 16 DTMF sounds. The information is sentto the message encoder 116. The audio signal is further transmitted tothe speech encoder 110, which processes the speech, as described above.If the DTMF detector 802 has detected a DTMF sound, it sends a signal tothe message encoder 116 over connection 804. This signal makes themessage encoder 116 replace the 123 speech coding bits corresponding tothe message coding bits with a code word corresponding to the detectedDTMF sound. Otherwise the message encoder 116 will transmit the speechcoding bits to the channel 108 as such.

The message decoder 120 of the receiver 102 examines the 123 speechcoding bits selected for the message transmission. If the receiver 102detects one of the DTMF code words, it sends this information 806 forfurther operations. Instead of a separate received DTMF message 806 orin addition to it, the receiver 102 may synthesize a “pure” DTMF soundpair to the speech output 112 of the receiver. Since transmission errorsmay occur on the speech channel 108, the message decoder 116 mayinterpret the code word as detected, if the received word differs fromthe code word, for example, by less than 5 bits. The message decodertransmits the received bits to the speech decoder 907, which synthesizesthe audio sound 908 to be heard.

The method allows the receivers provided with a message decoder of theinvention to detect DTMF sounds reliably. Previously used receivers inwhich the method is not implemented do not benefit from DTMF signallingtransmitted among speech coding bits, but nor does the signallingdisturb them.

A third embodiment of the invention is the use of the method forcreating an interactive protocol. The method of the invention is wellsuited for forming an interactive protocol. Since one field of use ofthe invention is introducing new properties to an existingtelecommunication system, the communicating equipments must often agree,at the beginning of a speech connection and also during it, on themethods to be used. If, for instance, a new speech coding method is tobe used, the communicating equipments must at the beginning of thespeech connection have a dialogue, i.e. perform handshaking, duringwhich they can observe that it is possible to use the new speech codingmethod.

The communicating equipments in the telecommunication system are mainlytransceivers. The same equipment thus comprises both a transmitter and areceiver. In this case, creating a protocol means simply that after thereceiver of the transceiver has received a message, it informs thecorresponding transmitter of the message; the transmitter reacts to itby sending a “treply” to the received message. It is thus possible toaccomplish a dialogue between two transceivers. During such a dialogueit is possible to ask whether a new speech coding method can be used andto reply to the question.

The method of the invention thus makes it possible to plan protocols ofthis kind without disturbing the equipments that have been previouslyused in the system. If one of the communicating equipments is notprovided with a message transmission method of the invention, it cannotreply according to the protocol to another equipment, which can thusconclude that the new function (in this case, a new speech codingmethod) cannot be used during this speech connection.

The invention has been described by way of example with reference to theGSM system. It should, however, be understood that the embodiments andexamples described are in all respects illustrative, not restrictive.Corresponding parameters can also be found in many other digitaltelecommunication systems. It will be obvious to one skilled in the artthat, as the technology advances, the basic idea of the invention can beimplemented and the invention can be applied in many different ways.Thus the invention and the embodiments thereof are not limited to theexamples described above, but they can be modified within the scope ofthe appended claims.

Appendix 1 to Patent Application

“Method and apparatus for transmitting messages in a telecommunicationsystem”

TABLE 1 Parameter Parameter Parameter Variable Number Bit number groupnumber name name of bits (LSB-MSB) Short term  1 Log area ratio 1 LAP 16 1-6 filter parameters  2 Log area ratio 2 LAR 2 6  7-12  3 Log arearatio 3 LAR 3 5 13-17  4 Log area ratio 4 LAR 4 5 18-22  5 Log arearatio 5 LAR 5 4 23-26  6 Log area ratio 6 LAR 6 4 27-30  7 Log arearatio 7 LAR 7 3 31-33  8 Log area ratio 8 LAR 8 3 34-36 Sub-frame no. 1Long term  9 LTP lag N₁ 7 37-43 prediction parameters 10 LTP gain b₁ 244-45 RPE parameters 11 RPE grid position M₁ 2 46-47 12 block amplitudeX_(max1) 6 48-53 13 RPE-pulse no. 1 x₁(1) 3 54-56 14 RPE-pulse no. 2x₁(2) 3 57-59 . . . . . . . . . . . . . . . 25 RPE-pulse no. 13 x₁(13) 390-92 Sub-frame no. 2 Long term 26 LTP lag N₂ 7 93-99 predictionparameters 27 LTP gain b₂ 2 100-101 RPE parameters 28 RPE grid positionM₂ 2 102-103 29 Block amplitude X_(max2) 6 104-109 30 RPE-pulse no. 1x₁(1) 3 110-112 31 RPE-pulse no. 2 x₂(2) 3 113-115 . . . . . . . . . . .. . . . 42 RPE-pulse no. 13 x₂(13) 3 146-148 Sub-frame no. 3 Long term43 LTP lag N₃ 7 149-155 prediction parameters 44 LTP gain b₃ 2 156-157RPE parameters 45 RPE gain position M₃ 2 158-159 46 Block amplitudeX_(max3) 6 160-165 47 RPE-pulse no. 1 x₃(1) 3 166-168 48 RPE-pulse no. 2X₃(2) 3 169-171 . . . . . . . . . . . . . . . 59 RPE-pulse no. 13 X₃(13)3 202-204 Sub-frame no. 4 Long term 60 LTP lag N₄ 7 205-211 predictionparameters 61 LTP gain b4 2 212-213 RPE parameters 62 RPE grid positionM₄ 2 214-215 63 Block amplitude X_(max4) 6 216-221 64 RPE-pulse no. 1x₄(1) 3 222-224 65 RPE-pulse no. 2 x₄(2) 3 225-227 . . . . . . . . . . .. . . . 76 RPE-pulse no. 13 x₄(13) 3 258-260

TABLE 2 Importance classes Parameter name Parameter number Bit index(LSB = 0) Bit number 1 1a Log area ratio 1 1 5 6 5 bits 50 bits 12, 29,46, 63 5 53, 109, 165, 221 2 Log area ratio 1 1 4 5 3 bits Log arearatio 2 2 5 12 Log area ratio 3 3 4 17 3 Log area ratio 1 1 3 4, 31 bitsLog area ratio 2 2 4 11 Log area ratio 3 3 3 16 Log area ratio 4 4 4 122LTP lag (4 sub-frames) 9, 26, 43, 60 6 43, 99, 155, 211 Block amplitude(4 sub-frames) 12, 29, 46, 63 4 52, 108, 164, 220 Log area ratio 2, 5, 62, 5, 6 3 10, 26, 30 LTP lag (4 sub-frames) 9, 26, 43, 60 5 42, 98, 154,210 LTP lag (4 sub-frames) 9, 26, 43, 60 4 41, 97, 153, 209 LTP lag (4sub-frames) 9, 26, 43, 60 3 40, 96, 152, 208 LTP lag (4 sub-frames) 9,26, 43, 60 2 39, 95, 151, 207 4 Block amplitude (4 sub-frames) 12, 29,46, 63 3 51, 107, 163, 219 25 bits Log area ratio 1 1 2 3, Log arearatio 4 4 3 21 Log area ratio 7 7 2 33 LTP lag (4 sub-frames) 9, 26, 43,60 1 38, 94, 150, 206 1b Log area ratio 5, 6 5, 6 2 25, 29 128 bits LTPgain (4 sub-frames) 10, 27, 44, 61 1 45, 101, 157, 213 LTP lag (4sub-frames) 9, 26, 43, 60 0 37, 93, 149, 205 RPE grid position (4sub-frames) 11, 28, 45, 62 1 47, 103, 159, 215 5 Log area ratio 1 1 1 2123 bits Log area ratio 2, 3, 8, 4 2, 3, 8, 4 2 9, 15, 36, 20 Log arearatio 5, 7 5, 7 1 24, 32 LTP gain (4 sub-frames) 10, 27, 44, 61 0 44,100, 156, 212 Block amplitude (4 sub-frames) 12, 29, 46, 63 2 50, 106,162, 218 RPE pulses (sub-frame 1) 12-25 2 56, 59, 62, 65, 68, 71, 74,77, 80, 83, 86, 89, 92 RPE pulses (sub-frame 2) 30-42 2 112, 115, 118,121, 124, 127, 130, 133, 136, 139, 142, 145, 148 RPE pulses (sub-frame3) 47-59 2 168, 171, 174, 177, 180, 183, 186, 189, 192, 195 198, 201,204 RPE pulses (sub-frame 4) 64-76 2 224, 227, 230, 233, 236, 239, 242,245, 248, 251, 254, 257, 260 RPE grid position (4 sub-frames) 11, 28,45, 62 0 46, 102, 158, 214 Block amplitude (4 sub-frames) 12, 29, 46, 631 49, 105, 161, 217 RPE pulses (sub-frame 1) 13-25 1 55, 58, 61, 64, 67,70, 73, 76, 79, 82, 85, 88, 91 RPE pulses (sub-frame 2) 30-42 1 111,114, 117, 120, 123, 126, 129, 132, 135, 138, 141, 144, 147 RPE pulses(sub-frame 3) 47-59 1 167, 170, 173, 176, 179, 182, 185, 188, 191, 194197, 200, 203 RPE pulses (sub-frame 4) 64-67 1 223, 226, 229, 232 2 RPEpulses (sub-frame 4) 68-76 1 235, 238, 241, 244, 247, 250, 253, 256, 2596 78 bits Log area ratio 1 1 0 1 73 bits Log area ratios 2, 3, 6 2, 3, 61 8, 14, 28 Log area ratios 7 7 0 31 Log area ratios 8 8 1 35 Log arearatios 8, 3 8, 3 0 34, 13 Log area ratios 4 4 1 19 Log area ratios 4, 54, 5 0 18, 23 Block amplitude (4 sub-frames) 12, 29, 46, 63 0 48, 104,160, 216 RPE pulses (sub-frame 1) 13-25 0 54, 57, 60, 63, 66, 69, 72,75, 78, 81, 84, 87, 90 RPE pulses (sub-frame 2) 30-42 0 110, 113, 116,119, 124, 127, 128, 131, 134, 137 140, 143, 146 RPE pulses (sub-frame 3)47-59 0 166, 169, 172, 175, 178, 181, 184, 187, 190, 193 196, 199, 202RPE pulses (sub-frame 4) 64-76 0 222, 225, 228, 231, 234, 237, 240, 243,246, 249, 252, 255, 258 Log area ratios 2, 6 2, 6 0 7, 27

TABLE 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 2425 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 4849 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 7273 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 9697 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259260

TABLE 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 2425 26 27 28 29 30 31 46 47 49 50 51 52 53 55 56 58 59 61 62 64 65 67 6870 71 73 74 76 77 79 80 82 83 85 86 88 89 1 1 0 0 0 0 0 1 1 0 0 1 1 1 00 0 1 1 1 0 1 0 1 1 1 1 0 0 1 1 1 1 0 0 0 0 0 0 0 1 0 0 0 1 1 1 1 1 0 00 1 0 0 1 0 1 0 0 0 1 1 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 1 1 0 0 1 1 0 0 10 0 0 1 1 1 1 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 1 0 0 1 1 0 1 1 0 1 1 0 0 10 1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 1 0 1 0 1 1 1 1 0 0 0 1 0 0 0 0 1 1 0 00 0 0 0 1 0 0 1 1 0 1 0 1 1 0 1 1 0 1 1 1 1 1 1 0 1 0 0 1 0 0 0 0 0 1 10 1 0 1 0 1 0 1 1 1 0 0 1 0 1 1 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 1 01 0 1 0 0 1 0 1 1 1 0 1 0 1 0 0 1 0 1 0 0 0 0 0 1 1 1 1 0 1 0 0 0 0 1 01 1 1 1 1 0 1 0 0 1 1 0 1 1 0 0 0 0 0 1 1 0 1 0 1 0 0 0 1 0 0 1 1 0 0 00 0 0 0 1 1 1 1 1 0 0 0 0 0 0 0 1 1 0 0 1 1 1 1 0 0 0 0 0 1 1 0 0 0 0 10 1 1 0 0 0 0 0 0 0 1 0 1 1 1 0 0 1 1 1 0 0 1 1 0 1 0 0 1 0 0 1 1 1 1 00 0 0 0 1 1 1 1 0 1 0 1 0 0 1 0 0 0 0 1 1 1 1 1 1 0 1 0 0 0 0 0 0 0 0 01 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 0 0 1 0 1 0 0 0 0 0 0 0 1 1 1 0 0 00 0 0 0 0 1 0 1 1 0 1 0 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 0 1 0 1 1 1 1 1 00 0 0 1 1 1 0 1 1 1 1 0 1 32 33 34 35 36 37 38 39 40 41 42 43 44 45 4647 48 49 50 51 52 91 92 102 103 105 106 107 108 109 111 112 114 115 117118 120 121 123 124 126 127 0 0 1 1 0 0 0 0 0 0 1 1 0 1 1 0 1 1 1 1 0 11 0 0 0 0 0 0 0 0 0 1 1 0 0 0 1 1 0 1 0 1 1 1 0 0 0 0 0 0 1 0 0 1 0 1 11 1 1 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 1 0 1 0 1 1 0 0 0 0 0 10 1 0 1 1 0 1 1 1 1 1 0 0 1 1 0 0 0 0 0 0 1 1 0 1 1 1 1 1 1 1 0 0 0 1 10 0 0 0 0 0 0 1 0 1 0 0 0 1 0 1 1 1 1 1 1 0 0 0 0 0 1 0 1 0 0 0 1 0 1 10 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 1 1 1 0 1 1 1 0 0 0 0 0 0 0 1 00 1 1 0 1 0 1 1 0 0 1 1 0 0 0 0 0 1 1 1 1 0 1 1 1 1 0 0 0 1 0 1 1 0 0 00 0 1 1 0 0 1 1 0 1 0 0 0 1 1 1 0 1 0 0 0 0 0 0 1 1 1 1 0 0 1 1 1 0 0 01 0 1 0 0 0 0 0 0 0 1 1 1 0 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 1 0 1 1 0 1 11 0 0 1 0 0 0 1 1 0 0 0 0 0 1 1 1 1 0 1 1 1 1 0 0 0 1 0 1 1 0 0 0 0 0 11 0 0 1 1 0 1 0 0 0 1 1 1 0 1 0 0 0 0 0 0 1 1 1 1 0 0 1 1 1 0 0 0 1 0 10 0 0 0 0 0 0 1 1 1 0 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 1 0 1 1 0 1 1 1 0 01 0 1 1 0 0 0 0 0 0 0 1 1 0 0 1 1 0 0 1 0 0 0 53 54 55 56 57 58 59 60 6162 63 64 65 66 67 68 69 70 71 72 129 130 132 133 135 136 138 139 141 142144 145 147 148 158 159 161 162 163 164 0 1 1 0 1 0 0 1 0 0 1 1 1 1 1 00 0 0 0 1 1 0 0 1 0 1 0 0 1 0 1 1 0 1 1 0 0 0 0 0 1 0 0 1 0 1 1 1 1 0 01 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 1 0 0 1 0 0 00 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 0 1 0 1 0 0 1 1 0 1 1 0 0 0 0 0 0 1 11 0 1 1 1 0 0 0 1 1 0 0 0 0 0 0 0 1 0 0 1 1 0 0 1 1 1 1 1 1 1 0 0 0 0 01 1 0 0 0 1 1 0 0 1 1 1 0 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 0 1 0 0 0 0 1 00 0 0 0 1 1 0 1 1 0 0 0 1 0 0 0 1 1 1 1 0 0 0 0 0 1 0 1 0 0 1 1 0 0 1 11 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 0 1 0 1 1 0 0 1 0 0 0 0 1 0 1 1 0 1 0 11 1 0 0 0 1 1 0 0 0 0 0 1 1 1 1 0 1 1 1 0 0 1 1 1 0 1 0 0 0 0 0 0 1 0 11 0 0 1 0 1 0 1 1 0 1 0 0 0 0 0 73 74 75 76 77 78 79 80 81 82 83 84 8586 87 88 89 90 91 92 93 165 167 168 170 171 173 174 176 177 179 180 182183 185 186 188 189 191 192 194 195 0 0 0 1 1 0 0 1 1 1 0 0 0 0 0 1 1 11 0 1 0 0 1 0 1 1 0 0 1 0 1 1 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 1 1 1 1 00 1 1 0 0 1 1 0 0 0 0 1 1 1 1 0 1 1 1 1 1 0 0 1 0 1 0 0 1 0 0 1 1 1 1 01 1 0 1 0 1 1 1 1 1 0 0 1 1 1 0 1 0 1 1 1 1 0 0 0 0 1 1 0 0 1 1 1 1 0 00 0 1 1 1 1 0 0 1 1 0 0 0 1 1 1 0 0 1 0 1 0 0 0 0 1 1 1 1 1 0 1 0 0 0 01 1 0 1 1 1 0 0 1 0 1 0 1 0 1 1 0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 1 1 1 11 1 0 0 0 0 0 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 1 0 1 0 0 0 1 0 0 0 1 0 0 10 0 1 0 0 1 1 0 0 1 1 1 0 1 0 1 0 0 0 1 0 1 1 0 1 1 0 1 0 1 0 1 1 1 1 00 0 0 0 0 0 1 1 0 1 1 1 1 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 0 1 0 0 1 00 1 1 1 1 0 1 1 1 0 0 1 0 1 1 1 0 1 0 0 0 0 0 1 1 0 1 0 1 0 94 95 96 9798 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 197198 200 201 203 204 214 215 217 218 219 220 221 223 224 226 227 229 230232 233 0 0 1 0 1 1 1 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 1 1 1 0 1 1 0 0 0 00 0 0 0 0 1 1 1 1 0 0 1 0 1 0 1 0 1 0 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 00 1 1 0 0 0 0 0 1 0 1 0 0 0 1 0 1 0 1 1 1 0 0 1 0 0 0 0 0 0 0 1 1 0 0 11 1 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 1 1 0 1 1 0 1 0 1 1 1 1 0 0 0 0 0 00 0 0 0 0 1 0 1 0 1 1 1 0 0 1 0 0 0 0 0 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 10 0 0 0 0 1 1 0 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 1 1 1 0 1 1 0 1 0 01 1 1 1 0 1 0 0 0 0 0 1 0 0 0 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 1 0 0 00 0 1 1 1 0 1 1 0 1 1 1 0 0 0 0 0 1 1 0 0 0 0 1 0 1 1 1 0 0 1 0 0 0 0 00 0 0 1 1 1 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 1 0 0 1 1 1 10 0 1 0 0 0 0 0 0 0 1 0 1 1 0 1 115 116 117 118 119 120 121 122 123 236239 242 245 248 251 254 257 260 1 0 1 0 0 1 0 1 0 0 0 1 0 1 1 0 1 0 0 00 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 1 1 0 0 0 0 1 1 1 0 1 0 0 1 0 1 10 1 1 0 0 0 0 1 1 1 1 1 0 1 1 0 1 1 0 0 0 0 1 1 1 0 0 1 0 0 1 1 0 1 1 10 1 1 0 0 0 1 1 0 1 1 0 0 0 1 0 1 1 1 0 0 0 1 0 0 1 0 1 1 0 1 1 0 1 0 00 0 1 1 1 0 1 0 1 0 1 0 1 0 0 1

What is claimed is:
 1. A method for transmitting messages simultaneously with speech in a digital telecommunication system; said method comprising coding speech to speech frames which consist of bits and which are positioned in a transmission signal; and transmitting each message by making at least one change in said transmission signal; wherein the messages are transmitted in short sequences relative to the speech information in a speech frame; for each possible message, a corresponding predetermined bit group is defined; said at least one change in said transmission signal comprises replacing less than all of the bits of the corresponding speech frame with said predetermined bit group corresponding to the message; and said at least one change in said transmission signal is restricted in time to the transmission moment of the speech frame corresponding to said message such that no other indication of the presence of the message is required.
 2. A method according to claim 1, characterized in that said replacing the bits of speech frames with a predetermined bit group comprises: setting parameters corresponding to the intensity of the sound of the speech frame to be transmitted in such a way that the amplitude of the received speech frame is minimized; and positioning the bit group corresponding to the message in a part of the speech frame which is protected in the system against transmission errors.
 3. A method according to claim 2, characterized in that the method is used in connection with full-rate speech coding of a GSM-type system and: the amplitude of the speech frame is minimised by setting the block amplitude parameters of the speech frame to be transmitted substantially as zero; and the bit group corresponding to the message is positioned in the bits corresponding to the RPE parameters of the speech frame.
 4. A method according to claim 1, wherein the number of possible messages is 20 or less and the bit groups corresponding to the messages are selected such that the Hamming distance between them is as great as possible.
 5. A method according to claim 1, further comprising: monitoring any DTMF signals that may occur in the speech; and in response to the detection of a DTMF signal in the speech, generating a message corresponding to said DTMF signal.
 6. A method according to claim 5, further comprising preventing the propagation of a DTMF signal as a DTMF sound to the signal of the transmission channel.
 7. A method for receiving a message of short duration in a digital telecommunication system, said method comprising receiving speech frames, some of which contain a message, wherein: a corresponding predetermined bit group is defined per each possible message; and identification of speech frames containing messages is based only on examination of the contents of the speech frames; and identification of a speech frame as a message requires at least that said speech frame contains a bit group which corresponds to one of said messages, wherein the speech frames are interpreted as messages when the speech frame contains a bit group which differs from a predetermined bit group associated with a message by no more than a predetermined threshold value the bit group including less than all bits of the speech frame.
 8. A method according to claim 7, wherein the predetermined threshold value is five bits.
 9. A method according to claim 8, wherein identification of a speech frame as a message further requires that the parameters corresponding to the intensity of the sound of a speech frame are below a second predetermined threshold to make the speech signal inaudible.
 10. A method according to claim 7, wherein at least some messages correspond to different DTMF sounds, and if a speech frame is interpreted as a message corresponding to a DTMF sound, the DTMF sound corresponding to the message is generated.
 11. A method according to claim 7, wherein the speech frames that are concluded to contain messages are processed as messages and as normal speech frames.
 12. A method according to claim 7, wherein the speech frames that are concluded to contain messages are processed as messages and as bad speech frames.
 13. A transmitter of a digital telecommunication system, comprising a speech encoder for encoding speech to speech frames consisting of bits, and a message encoder for encoding messages to a signal of a transmission channel common to the speech and messages; said transmitter being arranged to make at least one change in the signal of the transmission channel in order to transmit each message simultaneously with speech; wherein the transmitter is further arranged to: transmit messages for short periods at a time relative to the time associated with a speech frame; code each message to a corresponding predetermined bit group; restrict said change in the signal of the transmission channel in time to the transmission moment of the speech frame corresponding to said message; and replace less than all of the bits of the speech frame corresponding to said message with the corresponding predetermined bit group associated with said message; whereby no other indication of the transmission moment of the message is required.
 14. A transmitter according to claim 13, wherein the speech encoder is implemented as a state machine to which a feedback signal is further supplied from the message encoder whereby the speech encoder is arranged to change state during transmission of messages such that the effect of the transmission of message on the quality of speech is minimized.
 15. A transmitter according to claim 13, wherein the transmitter is arranged to: set the parameters corresponding to the intensity of the sound of the speech frame to be transmitted in such a way that the amplitude of the received speech frame is minimized; and position the bit group corresponding to the message in the part of the speech frame which is best protected against transmission errors.
 16. A transmitter according to claim 15, wherein the transmitter is part of a GSM system, and the transmitter is arranged to: minimize the amplitude of the received speech frame by setting the block amplitude parameters of the speech frame to be transmitted below a predetermined threshold to make the speech signal inaudible; and position the bit group corresponding to the message in the bits corresponding to the RPE parameters of the speech frame.
 17. A transmitter according to claim 13, further comprising: a DTMF detector for detecting DTMF signals occurring in a speech signal and for informing the message encoder of such a detection; and the message encoder is arranged to generate a message corresponding to said DTMF signal in response to the detection of the DTMF signal.
 18. A transmitter according to claim 17, wherein the DTMF detector or the message encoder are further arranged to substantially prevent the propagation of a DTMF signal as a DTMF sound to the transmission channel.
 19. A receiver of a digital telecommunication system, comprising a speech decoder for generating speech from speech frames, and a message decoder for decoding messages wherein a corresponding predetermined bit group is defined per each possible message; and the receiver is arranged to: receive a signal comprising speech frames, some of which contain a message; identify the speech frames containing messages only on the basis of the contents of the speech frames; and monitor at least whether a speech frame contains a bit group which corresponds to one of said messages, and when it does, to interpret said speech frame as a message, the bit group including less than all bits of the speech frame.
 20. A receiver according to claim 19, wherein the message decode is arranged to supply messages from the speech frames that are concluded to contain messages to the speech decoder.
 21. A receiver according to claim 19, wherein the message decoder is arranged to supply the speech frames containing messages to the speech decoder through a bad parameter replacement block.
 22. A receiver according to claim 19, wherein at least some messages correspond to different DTMF sounds, and the receiver is arranged, in response to a speech frame being interpreted as a message corresponding to a DTMF sound, to generate the DTMF sound corresponding to the message.
 23. A transmitter according to claim 19, wherein the number of messages is less than 20, and the bit groups corresponding to the messages are selected such that the Hamming distance between them is as great as possible.
 24. A signal of a speech channel in a digital telecommunication system, wherein the speech to be transmitted is coded as speech frames, some of which contain a message, wherein the changes to be made in the signal of the speech channel in order to transmit a message are restricted in time to the transmission moment of the message, and that less than all of the bits of the said speech frame are replaced with a predetermined bit group corresponding to said message.
 25. A signal of a speech channel according to claim 24, wherein said changing further comprises that the parameters corresponding to the intensity of the sound of the speech frames corresponding to the transmission moment are below a predetermined threshold to make the speech signal inaudible.
 26. A signal of a speech channel according to claim 25, wherein the signal is used in connection with full-rate speech coding of a GSM system, and in order to improve the immunity to disturbance, the block amplitude parameters of the speech frame are below a predetermined threshold to make the speech signal inaudible, and the RPE parameters are bit groups other than zero in such a manner that a different bit group corresponds to each different message and the Hamming distance between the bit groups is as great as possible. 